What is it you expect the school to do? Turn up at her house, force their way in, strip her off and examine her for injuries? On a white horse in a suit of armour perhaps?
Schools can't just wade in and save children, there is a process they and SS have to follow to make sure any allegations stand up in court. That takes time. Without knowing what safeguarding process was followed, you can't possibly pin this on the DSL at the school.
And there already IS professional responsibility, teaching is one of the few jobs where you can be held legally liable for failure to report.
As for social services, they're dead in the water. They need more hands on deck and more resources and more time to investigate properly and less red tape and bureaucracy. It's nothing to do with "moral compass".
Maybe you should retrain as a teacher or child protection SW and see how you do at it.